There are many reasons for lying down dyspnea and sitting up to breathe smoothly, and the common reasons are acute left heart failure, pleural effusion and other diseases. 1. Acute left heart failure: heart failure caused by lung stagnation due to respiratory dysfunction of the lungs, due to the increase in the volume of return blood when lying down, the heart load is increased, dyspnea occurs, sitting up can reduce the burden on the heart, reduce the volume of return blood, in order to breathe smoothly, also known as sedentary respiration. 2. Pleural effusion: a small amount of pleural effusion may have no symptoms, a large amount of pleural effusion will compress the lung tissue, affecting the normal contraction of the lung tissue, dyspnea, sit up because of the effect of gravity of the effusion, the lung tissue pressure is relatively reduced, appearing to lie down dyspnea sitting up and breathing smooth symptoms. There may be other causes of dyspnea, so it is recommended to go to the hospital in time to find out the cause and standardize the treatment under the doctor’s guidance.